Refine the `Awaitable` abstraction #4160

Job	Run time
build-on-lower-bound (ubuntu-latest, 4.12.x)	4m 51s
test-on-windows (ocaml.5.0.0,ocaml-option-mingw)	19m 54s
test-on-windows (ocaml.5.1.1,ocaml-option-mingw)	20m 10s
test-on-windows (ocaml.5.2.0,ocaml-option-mingw)	21m 4s
	1h 5m 59s

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